1. Field of the Invention
This invention relates generally to broadband optical amplifier devices. More particularly, it relates to broadband optical amplifier devices based on combinations of gain-clamped semiconductor optical amplifiers (SOAs), such as vertically lasing semiconductor optical amplifiers (VLSOAs).
2. Description of the Related Art
Optical amplifiers, which boost the power of optical signals, are a basic building block for many types of optical systems. For example, fiber optic communications systems transmit information optically at very high speeds over optical fibers. A typical communications system includes a transmitter, an optical fiber, and a receiver. The transmitter incorporates information to be communicated into an optical signal and transmits the optical signal via the optical fiber to the receiver. The receiver recovers the original information from the received optical signal. In these systems, phenomena such as fiber losses, losses due to insertion of components in the transmission path, and splitting of the optical signal may attenuate the optical signal and degrade the corresponding signal-to-noise ratio as the optical signal propagates through the communications system. Optical amplifiers are used to compensate for these attenuations.
Fiber amplifiers are one type of optical amplifier. They include a length of fiber which has been doped to form an active gain medium. Ions of rare-earth metals, such as erbium, are typically used as the dopant. The doped fiber is typically pumped by an optical pump at a wavelength which is preferentially absorbed by the ions but different from the wavelength of the optical signal to be amplified. The pumping results in a population inversion of electronic carriers in the active medium. Then, as the optical signal propagates through the doped fiber, it is amplified due to stimulated emission.
One drawback of fiber amplifiers is that they typically can only operate over a narrow wavelength range, especially when multiple fiber amplifiers are cascaded. This is especially problematic if the optical signal to be amplified covers a wide range of wavelengths, as would be the case if the entire bandwidth of the optical fiber is to be efficiently utilized. For example, single mode optical fiber is currently used in many fiber optic communications systems due to its good performance, particularly for high data rates over long distances. Present-day single mode fiber has low attenuation over the 1200 to 1650 nm wavelength range and, therefore, could support the transmission of optical signals over this entire wavelength range. However, erbium doped fiber amplifiers (EDFAs), which are the current amplifiers of choice for such systems, have a gain profile that limits their use to the approximately 1530 to 1565 nm wavelength range. This EDFA limitation means that only approximately 10% of the single mode fiber""s inherent capacity is currently utilized. The other 90% lies unused. Considerable effort has been expended to increase the wavelength range of EDFAs. However, in the near future, it appears that these efforts will at best expand the useable wavelength window to about 100 nm, or 25% of single mode fiber""s inherent capacity. Other disadvantages of fiber amplifiers include their relatively large size, slow switching speed, power inefficiency, difficulties in mass producing them, and their high cost which makes them prohibitively expensive for many applications.
Semiconductor optical amplifiers (SOAs) are another type of optical amplifier. SOAs contain a semiconductor active region and an electrical current typically is used to pump the electronic population in the active region. An optical signal propagating through the active region experiences gain due to stimulated emission. Conventional SOAs are non-lasing. One problem with non-lasing semiconductor optical amplifiers is that the gain depends on the amplitude of the optical signal. This problem is the result of gain saturation, in which there are insufficient carriers in the conduction band to provide the full amount of gain to higher power signals. As a result, a strong optical signal will be amplified less than a weak signal and strong portions of the optical signal will be amplified less than weak portions. This results in distortion of the optical signal and also crosstalk between different optical signals propagating simultaneously through the system (e.g., at different wavelengths). This significantly limits the use of conventional SOAs in broadband applications.
Thus, there is a need for optical amplifier devices which can operate over a broad wavelength range. There is also a need for optical amplifier devices which are small in size, easy to manufacture and easily integrable with other components. There is also a need for optical amplifier devices which do not suffer from gain saturation.
In accordance with the present invention, a broadband semiconductor optical amplifier (SOA) device includes at least two gain-clamped SOAs having different spectral responses. The gain-clamped SOAs are coupled so that the broadband SOA device has a spectral response which is not attainable by any one of the gain-clamped SOAs. The gain-clamped SOAs may be coupled in different ways. In one implementation, they are coupled in parallel. In another, they are coupled in series. Other types of couplings, including more complex ones, are also possible. For example, the broadband SOA device may include a number of stages, each of which includes a number of gain-clamped SOAs.
In another aspect of the invention, the broadband SOA device has a passband which is broader than the individual passbands of the gain-clamped SOAs. In another implementation, the passband of one of the gain-clamped SOAs lies in the 1.3 micron fiber transmission window and the passband of another of the gain-clamped SOAs lies in the 1.55 micron fiber transmission window. Thus, the broadband SOA device can amplify optical signals in both transmission windows.
In one embodiment, the gain-clamped SOAs are vertically lasing semiconductor optical amplifiers (VLSOAs). In one variation, the VLSOAs are integrated on a common substrate. In another approach, they are contained in separate packages.
In further accordance with the invention, in a broadband SOA device including at least two SOAs, a method for amplifying an optical signal includes the following steps. The optical signal to be amplified is received. Each SOA is gain-clamped. At least portions of the optical signal propagate along amplifying paths in the gain-clamped SOAs. The amplification applied by each gain-clamped SOA has a different spectral response and the spectral response of the amplification applied by the broadband SOA device is not attainable by any individual gain-clamped SOA.
The present invention is particularly advantageous because the broadband SOA device can operate over a broader wavelength range than a single gain-clamped SOA. Since the broadband SOA device is based on semiconductors (as opposed to fibers, for example), the device is also small in size, potentially easy to manufacture using semiconductor fabrication technology, and easily integrable. The gain-clamping aspect of the individual amplifiers alleviates problems arising from gain saturation.